In a referendum this weekend, the people of the Crimean Peninsula decided that their region should join the Russian Federation as an autonomous territory. Meanwhile, the Russian Duma is considering a law that would make it easier to annex neighboring territories in which “people have expressed distinct will and wish to become part of Russia.” After Crimea, in other words, Russia may try to annex larger and more economically significant parts of eastern Ukraine, which also include substantial numbers of Russians and Russian speakers.

Russia’s moves might have come as a shock, but for millennia, conquests and annexations were the meat and potatoes of state building and international politics. The groundwork for the United States’ own rise to global predominance was laid by its manifest destiny policy of westward territorial expansion, including the conquest of the southwest and California and the annexation of Mexico’s breakaway republic of Texas.

But that kind of imperialism has become virtually unheard of since World War II. There are some notable exceptions, including India’s conquest of Portuguese Goa in 1961, China’s forceful seizure of disputed but marginal territory from India in 1962, and North Vietnam’s absorption of South Vietnam in 1975. A few other prolonged occupations have stopped short of formal annexations, such as Israel’s occupation of the West Bank and Gaza since 1967, Armenia’s wresting of Nagorno-Karabakh from Azerbaijan in the early 1990s, and Russia’s “liberation” of South Ossetia and Abkhazia from Georgia in 2008. But compared to previous centuries, territorial expansion has been rare.

There are several possible reasons. For one, the victors of World War II banned aggression in the new United Nations charter. And during the Cold War, the superpowers assiduously checked expansion within and between their rival camps. After the Soviet Union collapsed, the United States continued to block attempted conquests such as Iraq’s play for Kuwait and Serbia’s unsuccessful effort to take much of Bosnia.

Some -- from nineteenth-century British free traders to contemporary international relations theorists such as Richard Rosecrance and Stephen Brooks -- have argued that global trade has diminished the profitability of conquest. States, the argument goes, can obtain capital and goods and penetrate markets more easily through friendly economic ties than through conquest. Being dependent on far-flung trade, moreover, renders conquerors vulnerable to economic sanctions imposed by outside powers looking to punish or reverse aggression.

Other explanations for the decrease in empire building focus on relations between conquerors and conquered. Rosecrance, Stephen Van Evera, and other scholars contend that foreign rule over modern nationalistic societies nets out as a loss for conquerors, because it requires costly repression and eviscerates local economic dynamism. Still others, such as the political scientist Gil Merom, theorize that liberal democracies lack the stomach to repress politically hostile populations. And a standard assumption of democratic peace theory is that liberal democracies are especially loath to dominate other democracies.

Those factors apparently haven’t deterred Russia from annexing Crimea, and it isn’t even clear that they will prevent Russia from coming out ahead in any eastern Ukrainian land grab. Crimea is economically insignificant, but it possesses a major naval base in Sevastopol, which Russia currently leases from Ukraine. Eastern Ukraine, however, produces over half of Ukraine’s industrial output and 40 percent of its GDP, which is equivalent to about four percent of the entire Russian economy. Moreover, the Ukrainian economy has been expanding by an average of 3.6 percent each year over the past decade, a growth rate only one percent lower than Russia’s over the same period. Further, although Ukraine remains less developed than Russia, it is among the world’s top ten arms exporters and steel producers.

The contribution of an annexed eastern Ukraine to Russia’s economy would depend, of course, on Russia’s ability to govern, tax, and maintain economic growth in the region -- something rebellion, unrest, and emigration could make nearly impossible.

Crimea would certainly be the easiest territory for Russia to digest. Despite having a bitterly anti-Russian Tartar minority, it is the only region in Ukraine that is dominated by a majority Russian population. In a recent poll conducted by the political scientists Grigore Pop-Eleches and Graeme Robertson, only 30 percent of Crimean respondents identified Ukraine as their homeland. Most identify with their own region, rather than with Russia. But their outlook could be similar to that of the Abkhazians, who sometimes chafe at the Russian expansion but much prefer Russian protection to the Georgian alternative.

It is hard to say how ready the populations in Ukraine’s other eastern provinces would be to accept renewed Russian authority. On the one hand, between 60 and 70 percent of eastern Ukrainians (including Crimeans) speak Russian as their primary -- if not necessarily native -- language in the home. Many resent the Ukrainian parliament’s recent vote (which was quickly vetoed by its interim president) to maintain Ukrainian as the official language throughout the country. Further, in the last election, almost four-fifths of eastern Ukrainian voters pulled the lever for ousted President Victor Yanukovych. And a Russian geopolitical orientation is more than twice as popular there as is a European Union one.

At the same time, Pop-Eleches and Robertson’s poll found that only a third of eastern Ukrainians outside of the Crimean Peninsula identify as ethnically Russian, and that nearly 70 percent named Ukraine as their homeland. Still, given the eastern Ukraine’s linguistic and political divergence with western Ukraine, their Ukrainian identity may not be so rigid.

It bears remembering that, in the Soviet era, Moscow did not flinch from using harsh means to consolidate authority in Ukraine. And it developed the republic into a major center of arms production and high-technology industry, despite Ukrainian resentment about the millions of deaths caused by Soviet leader Joseph Stalin in the early 1930s. Of course, contemporary Russia is much less authoritarian than was the Soviet Union. But Russian President Vladimir Putin has demonstrated a willingness to stifle political dissent throughout Russia, and might be able to do so in eastern Ukraine as well. The theory that liberal democracies cannot bear to conquer each other does not apply to illiberal regimes like Putin’s.

Other factors that are assumed to have rendered modern conquests unprofitable seem similarly inapplicable. No foreign power wants a military showdown with Russia over Ukraine. U.S. President Barack Obama has threatened that “there will be costs” to Russia’s violation of Ukrainian sovereignty. But the proposed penalties -- cancelling an upcoming U.S.-Russian summit, moving U.S. warships into the Black Sea, ejecting Russia from the G-8, or refraining from deepening U.S.-Russian trade ties -- are rebukes more than punishments with bite.

Tough multilateral economic sanctions also seem unlikely. Russia is a member of the World Trade Organization, which guarantees very low trade barriers among its members. The treaty grants exemptions allowing member states to impose arms embargoes when necessary for the protection of their “essential security interests” and any kind of sanction mandated under the UN Charter for the maintenance of international peace and security. But Russia will veto any Security Council resolutions challenging Russian aggression toward Ukraine. And many of the world’s largest states are unlikely to forego their imports of Russian oil and gas to punish Russia. Western Ukraine in particular would suffer economically if it tried interfering with Russian gas pipelines running through Ukraine to Europe.

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Putin can take comfort from the fact that there was little international reaction to Russia’s 2008 invasion of South Ossetia and Abkhazia and the generous 49-year leases on military bases Russia acquired there. The world hasn’t even blinked at Georgia’s recent complaints about Russian and Ossetian troops taking yet more territory along the boundary line between Tbilisi-administered region and South Ossetia. Putin could diffuse international opposition to Russian moves against greater eastern Ukraine by following the same playbook: incrementally subverting Ukrainian sovereignty with a “temporary” occupation, then encouraging citizens to request Russian annexation, and finally acceding to those requests.

Even a successful Russian absorption of eastern Ukraine would not disprove the claim that foreign rulers cannot repress nationalistic societies while also growing their economies. As noted above, the region’s sizable “fifth column” -- in this case the many Russians and Russophones -- of potential collaborators would facilitate Russia’s task. Similarly, Hong Kong’s Chinese population might account for the remarkable economic growth rates it has maintained after being annexed by China in 1997.

In short, the factors that should have deterred Putin in Ukraine did not; those that should prevent him from making gains there might not. This conclusion is worrisome for Ukrainians hoping to preserve their country’s sovereignty and territorial integrity -- and for those who fervently wish that conquest can no longer pay.